The technical use of thermoplastics has certain limitations, among other things, in terms of the material properties of the plastics. Probably, the most significant of these limitations is the restricted barrier effect that these plastics offer against gases, vapors and solvents. One possibility to somewhat improve or else overcome this limitation is gas-phase fluorination. With gas-phase fluorination, a polar barrier layer is formed on polyethylene or other polyolefins by means of brief exposure to a treatment gas containing fluorine. Such a process is described, for example, in German patent DE-PS 35 11 743. As a result of such treatment, the usefulness of these mass-produced plastics is considerably enhanced. Nevertheless, the processes for creating barrier layers known so far such as, for instance, co-extrusion, incorporation of barrier layer resins and coatings, are effective only against specific permeating substances, that is to say, certain contents.
As a rule, such barrier layers fail to a greater or lesser extent when they come into contact with mixtures of non-polar or strongly polar substances. The reason for this not yet fully understood phenomenon probably has to do with the swelling behavior of these barrier layer plastics with respect to such solvents. The polar fractions of the permeating substances cause a swelling of polar barrier layers which, in turn, also become more permeable to non-polar components. Naturally, the same applies to polar barrier layers which are covered on the medium side by a non-polar layer, for instance, as is the case with co-extrusion. In this context, first the non-polar outer layers are swollen by non-polar solvents, as a result of which they also become permeable to polar components. As a result of this "entrainment", polar solvents reach the area of the covered polar barrier layer and lead to a pronounced reduction of the barrier effect there, too.
Technical preparations usually consist of such complex admixtures of polar and non-polar substances. These are primarily products of the chemical and petrochemical industries such as, for example, fuels.
The invention is based on the objective of creating a process for the production of barrier layers on the inner surfaces of hollow molded parts made of thermoplastics, which are effective with respect to admixtures of polar and non-polar substances.
The invention is characterized by having the temperature of the surface at the beginning of exposure lie in the range of 60.degree. C. to 250.degree. C. [140.degree. F. to 482.degree. F.]. The reaction energy released during fluorination is removed by cooling the treatment gas.
In the preferred practice of the invention the treatment gas containing fluorine enters the hollow molded part at a temperature between 0.degree. C. and -196.degree. C. [32.degree. F. and -320.8.degree. F.]. The treatment gas is cooled by mixing it with low-temperature nitrogen which had been previously introduced into the molded part.